Device for measuring liquid flow rate

ABSTRACT

A device for measuring liquid flow rate in a duct by introducing a fluid bubble in the duct and measuring the interval between the bubble&#39;&#39;s passage of two spaced measuring points. To form and introduce the bubble there are provided two chambers in the duct so that the static pressure in the downstream chamber exceeds that in the upstream chamber. The chambers are connected by means of a separate bubble fluid duct opening into a downwardly facing recess in the upstream chamber whereby a fluid bubble is gradually formed and released to the liquid flowing through the chamber.

United States Patent DEVICE FOR MEASURING LIQUID FLOW RATE 7 Claims, 2Drawing Figs.

0.8. CI 73/194 R, 1 128/205 F Int. Cl. G011 1/00 Field of Search 73/194;128/205 [56] References Cited UNITED STATES PATENTS 3,308,660 3/1967 DeFord 73/194 3,333,362 6/1967 Wells 73/194 FOREIGN PATENTS 831,610 2/1952Germany 73/194 Primary ExaminerCharles A. Ruehl I Attorney-Stevens,Davis, Miller and Mosher ABSTRACT: A device for measuring liquid flowrate in a duct by introducing a fluid bubble in the duct and measuringthe interval between the bubbles passage of two spaced measuring points.To form and introduce the bubble there are provided two chambers in theduct so that the static pressure in the downstream chamber exceeds thatin the upstream chamber. The chambers are connected by means of aseparate bubble fluid duct opening into a downwardly facing recess inthe upstream chamber whereby a fluid bubble is gradually formed andreleased to the liquid flowing through the chamber.

DEVICE FOR MEASURING LIQUID FLOW RATE This invention relates to a devicefor measuring the flow a fluid having a lower specific gravity than theliquid and being immiscible therewith and measuring the passage time ofthe bubble between two fixed measuring points spaced along the duct.

When measuring the flow rate of very delicate or sterile liquids, e.g.in the extra-corporal blood circuit in a hemodialysis apparatus, thebubble method has the advantage over other measuring methods, such asthe electrodynamic method or the Doppler-effect method, that the liquiddoes not contact any measuring surfaces which may be difficult tosterilize or which may cause undesired precipitation of constituentsfrom the liquid, e.g. fibrin from blood which as known may cause theblood to coagulate. On the other hand, the method has the inconveniencethat it is difficult to introduce the bubble in the liquid in such a waythat it fills the cross section of the duct completely. The previouspractice has been to introduce an amount of air in the duct manually bymeans of a probe or cannula, but it is difficult to avoid splitting theamount of air into several bubbles which are too small to fill the ductsection completely, and it is also necessary to interrupt the liquidflow during the introduction of the air which changes the stationaryflow that should be measured.

The inconveniences pointed out are remedied in the device according tothe present invention which is characterized in that a first and asecond chamber each having a larger crosssectional area than the ductare connected in the duct upstream and downstream, respectively, of themeasuring point, the second chamber being located at a lower level thanthe first chamber, and in that the first chamber comprises a liquidinlet, a liquid outlet located at a higher level than the inlet and adownwardly facing recess laterally adjacent the outlet, the bottom ofsaid recess having an opening which is connected to the upper part ofthe second chamber through a separate duct.

Through a device according to the invention it is possible, withoutdisturbing the liquid flow, to carry out a continuous formation of fluidbubbles and introduction of these bubbles in the liquid in such a waythat each bubble completely fills the duct area in the region of themeasuring points. From the second or downstream chamber the bubble fluidwhich in that chamber is separated from the liquid being measured due tothe difference in specific gravity, flows continuously back to the firstor upstream chamber in which the fluid collects in the form of asteadily increasing bubble in the recess, until the bubble has reachedsuch a size that it escapes below the edge of the recess under theinfluence of the positive static differential pressure and continuestogether with the liquid past the measuring point. Since the same amountof fluid circulates continuously in the apparatus it is only necessaryto provide a suitable amount of fluid in the system, the balance ofwhich is liquid filled, and such amount of fluid may be introduced bymeans of a probe or cannula before the measuring operation commences.The device has a very simple structure and can therefore be produced atsuch low price that it may be thrown away after use, and if necessary,it can easily be sterilized, e.g. by radiation, before being used.

In order to facilitate the formation of a fluid bubble, the lower edgeof the recess may be substantially circular when viewed in plan.

The diameter of the recess may be approximately half as large as thediameter of the first chamber at the same horizontal level. In that casethe bubble leaving the recess has such a size that it can fill theoutlet cross section adjacent the recess completely.

It has proved expedient to provide the edge of the recess with asmoothly rounded cross sectional profile. The magnitude of the radius orcurvature of the edge which influences the stability of the bubble,depends inter alia upon the surface tension in the boundary face betweenthe bubble fluid and the measuring liquid and upon the diameter anddepth of the recess.

The cross-sectional area of the opening in the bottom of the recess maybe substantially smaller than the area of the separate duct connectedthereto. In that case the liquid is effectively prevented from flowinginto the separate duct.

In the duct connecting the first and second chambers a variablethrottling means may be provided, so that it is possible to control thefrequency of the bubble formation by changing the flow rate of thebubble fluid in the separate duct.

The first or upstream chamber may comprise a downwardly tapering lowerpart in the bottom of which the liquid inlet is provided, and an upperpart including an outlet passage tapering in substantially verticalupward direction and said recess, the lower edge of which is locatedsubstantially in the region of the maximum cross-sectional area of thechamber. The embodiment ensures a suitably low liquid flow rate in theregion of the chamber, in which the bubbles are formed and entrained bythe liquid so that the bubble is less liable to break up into smallerbubbles. In the tapering outlet the liquid is accelerated to its normalflow rate, and at the same time the bubble is constricted laterally toensure that it fills out the duct cross section completely.

The invention is described in more detail below with reference to theaccompanying drawing, in which FIG. 1 is a vertical section through adevice embodying the present invention as well as part of the associatedmeasuring equipment, and

FIG. 2 is a horizontal section along line A-A in FIG. I.

The device illustrated in the drawing is intending for measuring theflow rate in the flood circuit of a hemodialysis apparatus, in which apatients blood is purified by flowing along a diaphragm, while asuitable dialysis liquid flows along the opposite side of the diaphragm.The device comprises a container I composed substantially of two conicalor tapering portions joined in a horizontal center plane. In the bottomof the container 1 there is an inlet 2 which can be connected to a hosenot shown, through which the blood flows into the device in thedirection of arrow 3. In its upper part the container 1 has an eccentricoutlet 4 which via a transparent hose 5 is connected to the upper partof a second container 6 having an outlet 7 for the blood flow.

In two measuring points suitably spaced along the hose 5 there areprovided two light sources 8 with associated collimators 9 and detectors10. As described in more detail below, as air bubble is introduced atsuitable intervals in the liquid flowing through container 1, and whenthe air bubble passes the two measuring points it produces a signal inthe respective detectors 10. The time interval between the two signalsis consequently a representative of the liquid flow rate in the hose 5.

As shown in FIG. I the container 6 is arranged at a lower level than thecontainer 1, and the height difference is such that the staticdifferential pressure resulting therefrom ex- "ceeds the pressure dropdue to the liquid flow from container 1 to container 6. Container 6 hassuch large volume that the interior of the container is not completelyoccupied by the liquid flowing therethrough, so that an air space 11 ispresent above the liquid level in container 6. Via a hose 12 the airspace 11 is connected to the upper part of container 1 adjacent itsoutlet 4. At this place the wall of container 1 has an enlargement 13with a vertical bore to which the hose 12 is connected. The lowerportion 14 of the bore is restricted as compared to the hose l2 andopens into the plane bottom surface of a recess 15, the circumference ofwhich is defined by a bead 16 which is circular in plan view, see FIG.2, and the downwardly facing edge of which has a rounded cross section.

When the liquid flowing through the device contains a certain amount ofair which fills the air space 11 in container 6 and the hose 12, theabove mentioned static differential pressure between containers 6 and 1will cause a flow of air upwardly from space 11 through hose l2 and bore14 to the recess 15. The flow velocity of the air can be controlled bymeans of an adjustable throttling member 17 mounted on the hose. Themember 117 may be a simple clamp for mechanically squeezing or pressinghose 12 together to a smaller or larger extent. Due to the presence ofthe bead 16, the amount of air flowing into the recess 115 cannotimmediately continue into the liquid flowing through container 1, but itwill collect to form a steadily increasing bubble, as shown by 18 inFIG. 1. When the bubble has reached a size, so that it substantiallyfills out the recess l5 and begins to grow down below the edge of bead16, which is possible due to the above mentioned positive pressurewithin the bubble, it will-at a certain momentbe entrained by the liquidflow and escape sidewise below the edge of bead 16. When the bubble isclear of the recess it follows the liquid upwardly through outlet 4, atwhich its cross section is gradually restricted or decreased, as shownat 18' in FIG. 1. In FIG. 11 there is also shown a bubble 18" whichcompletely fills out the cross section of the hose 5 in the region ofthe detectors at the two measuring points.

The dimensions of the recess and the profile or contour of the definingbead 116 will depend inter alia upon the surface tension in the boundarysurface between the bubble and the liquid. A suitable rounding orradiusing of the bead edge has in particular proved expedient in orderto avoid the risk of breaking the bubble up into several smallerbubbles, when it escapes from the recess to the liquid flowing throughcontainer ll. As mentioned the frequency of the bubble formation can becontrolled by means of the throttling member 17. The double taperingshape of the container 1 further reduces the risk of the bubblebreaking, because the flow rate or speed of the liquid is relatively lowat the region around the maximum cross section of the container, inwhich the bubble is introduced in the liquid.

The device shown can readily be made entirely of plastic materials, andmay be sterilized by radiation before use. Since the device can beproduced at a very low price, there are no objections against throwingit away after use, especially if it is used in connection with ahemodialysis apparatus. A device according to the invention can also beutilized for other measurements of liquid flow. The bubbles may beformed by air or any other gas, which is compatible with the liquid,e.g. argon. For certain applications the bubbles may possibly be formedof a liquid which does not mix with the liquid, the flow rate of whichis to be measured, and which is specifically lighter than said liquid.Such a device may be constructed similar to that shown and comprising achamber or container in which the bubbles are formed, and a secondchamber located downstream of the first chamber, whereby the lighterliquid may be separated from the measuring liquid in the second chamber.In that chamber there will prevail a positive pressure sufficient fordriving the bubble fluid back to the bubble chamber and ensure theformation and introduction of the bubbles into the liquid. in theembodiment shown, the outlet from the bubble chamber extends verticallyupwards but this is no absolute condition, since the outlet may have asmaller inclination with respect to the horizontal plane.

What we claim is:

l. A device for measuring the flow rate of a liquid flowing through aduct, comprising two measuring points spaced along said duct, means forinitially introducing a gas having a lower specific gravity than saidliquid and being immiscible therewith into said device, means formeasuring the passage time of a bubble of said gas between saidmeasuring points, means defining a first chamber having a largercross-sectional area than said duct, said first chamber having a liquidinlet, a liquid outlet located at a higher level than said inlet andconnected to the upstream end of said duct, said upstream duct endextending initially in a generally upward direction from said liquidoutlet, a downwardly facing recess laterally ad- 20 jacent said outletand an opening in the bottom of said recess,

means defining a second chamber located at a lower level than said firstchamber and having a larger cross-sectional area than said duct, saidsecond chamber having a liquid inlet connected to the downstream end ofsaid duct, a liquid outlet and a gas outlet in the upper part of saidsecond chamber, and a gas duct connecting said gas outlet with saidopening in said recess of said first chamber.

2. A device as claimed in claim 11, wherein the lower edge of saidrecess is substantially circular when viewed in plan.

3. A device as claimed in claim 2, wherein the diameter of said recessis approximately half as large as the diameter of said first chamber atthe same horizontal level.

4. A device as claimed in claim ll, wherein the edge of said recess hasa smoothly rounded cross-sectional profile.

5. A device as claimed in claim 1, wherein the cross-sectional area ofsaid opening in the bottom of said recess is substantially less than thearea of said gas duct connected thereto.

6. A device as claimed in claim I, wherein a variable throttling meansis provided in said gas duct connecting said first and second chambers.

7. A device as claimed in claim 11, wherein said first chamber comprisesa downwardly tapering lower part and an upper part, said liquid inletbeing provided in the bottom of said lower part, and said upper partincludes an outlet passage tapering in substantially vertical upwarddirection towards said liquid outlet, the lower edge of said recessbeing located in said upper part substantially in the region of themaximum cross-sectional area of said first chamber.

1. A device for measuring the flow rate of a liquid flowing through aduct, comprising two measuring points spaced along said duct, means forinitially introducing a gas having a lower specific gravity than saidliquid and being immiscible therewith into said device, means formeasuring the passage time of a bubble of said gas between saidmeasuring points, means defining a first chamber having a largercross-sectional area than said duct, said first chamber having a liquidinlet, a liquid outlet located at a higher level than said inlet andconnected to the upstream end of said duct, said upstream duct endextending initially in a generally upward direction from said liquidoutlet, a downwardly facing recess laterally adjacent said outlet and anopening in the bottom of said recess, means defining a second chamberlocated at a lower level than said first chamber and having a largercross-sectional area than said duct, said second chamber having a liquidinlet connected to the downstream end of said duct, a liquid outlet anda gas outlet in the upper part of said second chamber, and a gas ductconnecting said gas outlet with said opening in said recess of saidfirst chamber.
 2. A device as claimed in claim 1, wherein the lower edgeof said recess is substantially circular when viewed in plan.
 3. Adevice as claimed in claim 2, wherein the diameter of said recess isapproximately half as large as the diameter of said first chamber at thesame horizontal level.
 4. A device as claimed in claim 1, wherein theedge of said recess has a smoothly rounded cross-sectional profile.
 5. Adevice as claimed in claim 1, wherein the cross-sectional area of saidopening in the bottom of said recess is substantially less than the areaof said gas duct connected thereto.
 6. A device as claimed in claim 1,wherein a variable throttling means is provided in said gas ductconnecting said first and second chambers.
 7. A device as claimed inclaim 1, wherein said first chamber comprises a downwardly taperinglower part and an upper part, said liquid inlet being provided in thebottom of said lower part, and said upper part includes an outletpassage tapering in substantially vertical upward direction towards saidliquid outlet, the lower edge of said recess being located in said upperpart substantially in the region of the maximum cross-sectional area ofsaid first chamber.